In the aircraft industry, sandwich panels of a type having a honeycomb core sandwiched between two outer skins are widely used. One such type of panel is a crushed core sandwich panel in which the core and skins are compressed and portions cured following their assembly. A “crushed core” process can be used for contoured panels and may or may not fully or partially crush the honeycomb core of the panel. Flat panels typically do not use the “crushed core” process, but rather can be made either by placing panel components in layers on a flat plate and then vacuum bagging the assembly and curing under heat and pressure, or by using a pair of plates that hold the flat panel together under some pressure and temperature while the panels cure. Typically, panel skins contain a resin that flows when the temperature is raised and adhere the panel components together.
During manufacturing of contoured or flat panels, heat is applied to cure the panel components, and the skins are bonded to the core and have smooth outer surfaces in the finished panel. When these panels are used in areas visible to airplane passengers, the finished appearance of the panel is important. Therefore, it is desirable to provide the finished panels with a decorative coating or covering to enhance their aesthetic appearance. When the core of the panel is a material such as honeycomb, the covered or coated edges of the panel have a rough uneven appearance if the honeycomb extends all the way to the edges. To avoid this uneven appearance, the edge portions of the core of the panels are formed by a relatively homogeneous material instead of honeycomb. Foam can also be used to increase strength on the panel edges.
Sandwich panels are used extensively in aircraft component construction due to their high strength to weight ratios. Typically, depending on use of the panel, additional components and inserts are used to secure sandwich panels to other sandwich panels, and also to attach other components to sandwich panels. An insert is a component that typically is installed into a hole feature cut into the sandwich panel and is held in place using an adhesive referred to as a “potting compound”. Inserts typically accept threaded items although a variety of engagement features exist, such as keyhole slots, undercuts, etc. Thus, sandwich panel inserts that utilize a potting compound (i.e., an adhesive) to bond the insert to sandwich panels require time to cure, during which no work can be performed on the inserts. Potting compound and insert are usually allowed to cure between 2 to 4 hours to reach sufficient strength to allow installation of features into the inserts.
Sandwich panel inserts may not hold their position well on a sandwich panel while curing or when subjected to accelerated cure processes. Forces due to potting compound expansion can shift an insert position during the cure process. Inserts that are shifted from their nominal location can result in aesthetic mark-off. At times, to maintain position of the insert during cure, masking tape is used to prevent mark-off. Furthermore, in instances in which inserts are installed into both sides of the panel, more time and effort is required to maintain positions of the inserts during cure.
In instances in which two panels are joined together, sometimes bonded panel joints are used. Multiple bonded joints for a larger assembly are typically completed at the same time, and then are clamped and held until curing has completed. This limits the ability to work on the panel apparatus until the curing is complete.
Using potting compounds and bonding of panel joints thus adds to the cost of assemblies through material costs as well as cycle times associated with curing.